Apply data validation to the cells:
In Excel 2003 (slightly different in Excel 2007), from the Menu - Data - Validation... Then in the dialog box select the Settings Tab, and in the Allow: drop-down select Decimal.
Enter two values in the Minimum and Maximum boxes.
This will limit data entry to numbers in your range - if lowest value is zero it will stop negative numbers being entered and text (that isn't a number) cannot be entered.
In the Validation dialog, once you have entered Min and Max values you can create a custom alert warning in the Error Alert Tab - this shows if the entry fails the validation test.
You could also add a pop-up message that shows whenever the cell is selected - e.g. Enter dollar amount without the $ sign.